Activity of the Novel Succinate Dehydrogenase Inhibitor Fungicide Pydiflumetofen Against SDHI-Sensitive and SDHI-Resistant Isolates of Botrytis cinerea and Efficacy Against Gray Mold

Succinate dehydrogenase inhibitor ('SDHI) fungicides are currently the most frequently used fungicides for controlling gray mold. However, isolates of Botrytis cinerea resistant to SDHI fungicides have emerged in the field. Pydiflumetofen is a new SDHI fungicide that can control a variety of fungal diseases, but its efficacy against gray mold and whether the activity of pydiflumetofen is affected by the current SDHI-resistant isolates is currently unknown. The sensitivity of 291 single-spore B. cinerea isolates collected from 2017 to 2019 to pydiflumetofen was determined by spore germination inhibition assays. The mean EC50 value (fungicide concentration resulting in a 50% inhibition compared with that of the control) of pydiflumetofen was 0.06 +/- 0.01, 0.07 +/- 0.02, and 0.05 +/- 0.02 mg/liter in 2017, 2018, and 2019, respectively. There was no significant difference in the sensitivity of B. cinerea to pydiflumetofen among the 3 years. Furthermore, pydiflumetofen at 300 mg/liter effectively controlled gray mold on cucumber leaves (80.9%), and its efficacy was superior to that of boscalid at 400 mg/liter (42.7%). The isolates carrying P225F, N230I, H272Y, and H272R mutations in the SdhB subunit were associated with the less sensitivity of B. cinerea to SDHI fungicides. After establishing the baseline sensitivity of B. cinerea to pydiflumetofen (EC50 of 0.03 +/- 0.003 mg/liter), we found that the P225F and H272Y mutant isolates showed low to moderate levels of resistance to pydiflumetofen, and the H272R and N230I mutant isolates showed low levels of resistance. The reduced sensitivity to pydiflumetofen resulted from the positive correlation of pydiflumetofen with the other four SDHI fungicides (i.e., boscalid, fluopyram, isopyrazam, and benzovindiflupyr). These results suggest that pydiflumetofen provides effective control for the management of gray mold but must be used with caution.